A major problem with ground-based radar systems has always been ground clutter. “Ground clutter” refers to radar reflections from structures located close to the radar transmitter-receiver, which are strong relative to ordinary or desired targets (generally aircraft or other vehicles) because of the short path lengths between the transmitter-receiver and the clutter. In many cases, the clutter arises from immobile objects, and simple discrimination against nonmoving targets suffices to ameliorate the clutter problem. Such discrimination is provided by “moving target indication,” which discriminates in favor of target signals which change as a function of time. It has been found, however, that even immobile objects can have some motion, as for example the movement of the leaves of trees has been known to occasion clutter, even when a moving target discriminator is used.
When a radar system is operated adjacent to a reflective surface such as the Earth or a sea, target signals arrive at the receiving antenna directly from the target, and also arrive in the form of reflections from the reflective surface. Since the arriving signals originated from or were reflected by the target, both the direct (non-ground-reflected) and ground-reflected signals have the changing electrical parameters characteristic of moving targets, and so moving target discrimination (MTI) is totally ineffective in separating the direct target signal from the ground-reflected target signal. Since the ground-reflected signal arrives at the receiving antenna from an elevation angle which is different from the angle at which the direct target signal arrives, and cannot be discriminated against by moving target indication, the apparent elevation angle of arrival of the composite signal (the sum of the direct target signal and the ground-reflected signal) is different from the elevation angle of arrival of the desired direct target signal alone. Since a major aspect of radar operation is the determination of target direction from the angle at which the target-reflected signal arrives at the receiver, an error in the apparent direction of arrival may be translated into an error in the apparent direction of the target.
In the context of a ground-controlled approach radar or its modern equivalents, the radar returns indicate both the angle of the target relative to the receiving antenna and its range. The range and angle are then processed using simple trigonometric identities to determine the altitude of the target. More generally, the trigonometric identities can be applied to determine the three Cartesian coordinates (e.g., x—downrange, y—cross-range, and z—height) of the target in an arbitrary earth-fixed coordinate system, typically but not necessarily centered on the radar. If the indicated elevation direction is incorrect, the calculated altitude will also be in error. The directional and range information can be important in navigation of the vehicle, and significant errors are undesirable. A method to reduce the effects of ground reflections on terrestrial radar systems is to place a physical wall (i.e., multipath fence) before the radar antenna, of a height insufficient to interfere with a direct path extending between the target and the radar antenna, but sufficiently high to at least partially block some of the target signals which might be reflected from the ground into the radar antenna. Such walls are expensive and inconvenient.
Many radar systems use “monopulse” techniques to aid in determination of the exact angular location of the target relative to two orthogonal axes lying in the plane of the array face. Typically, one axis is parallel to the array's element rows, the other is parallel to the array's element columns, and they intersect at the center of the array. Furthermore, the element rows are horizontal in conventional ground-based radars. In monopulse operation, a conventional single-main-lobe “sum” beam is directed toward the target. The beamwidth of the sum beam is sufficiently large that the target is unlikely to leave the peak region of the beam during ordinary maneuvers over the period of time required by the radar to collect sufficient signal for a monopulse angle estimate. The peak of such a sum beam, however, is relatively broad, and the signal reflected from the target is not particularly sensitive to slight variations of the target within the beam peak. Monopulse systems also make use of simultaneous and collinear “difference” beams in at least one of horizontal and vertical directions to refine the directional information. Generally, this information is in the form of angular magnitudes and senses relative to either one axis or two orthogonal axes lying in the plane of the array face. A difference beam is characterized by a sharp gain null, generally coincident with the beam peak of the sum beam. Furthermore, the phase of the difference beam changes by 180 degrees as the target angle traverses the null, thus allowing the radar to determine the sense of the target's deviation from the monopulse axis or axes as well as the deviation's magnitude. In the case of an array antenna, a difference beam can be generated by subtracting one beam portion from another. One simple way to accomplish this is to subtract the signals produced by two different and non-collinear beams of the array. More particularly, a very simple way to produce a difference beam is to take the difference between the target signals produced by two halves of the array operated independently of each other. An antenna including an array of antenna elements organized into rows and columns can be divided horizontally into two halves, and vertically into two halves, to thereby define four quarter-array sections. All four of the quarter-array sections are operated in a sum mode to generate the sum monopulse beam, the two upper quarter-array sections are summed and this sum operated in difference with the sum of the two lower quarter-array sections to produce the elevation difference beam, and the two right quarter-sections are summed and operated in difference with the sum of the two left quarter-sections to produce the azimuth difference beam. The two difference beam signals, when divided by the sum beam signal to eliminate the influence of absolute target signal magnitude, then give an accurate indication of the actual angular position of the target relative to the two previously discussed orthogonal axes.
One of the characteristics of antennas is that, in general, for antenna apertures (which may be viewed as the size relative to the wavelength) larger than a few wavelengths, the beamwidth (the angular width of the central high gain lobe) is inversely related to the aperture width. The sidelobe level, however, is related not to the aperture width but rather to the amplitude weighting applied across the aperture. For example, regardless of the size of the aperture, the peak sidelobe gain will always be 13 dB below the peak mainlobe gain if the aperture is linearly weighted. In order to reduce the peak sidelobe level, an amplitude taper such as Taylor weighting is applied across the aperture. This bell-curve-shaped taper emphasizes the contribution of signals incident near the center of the aperture and smoothly decreases the emphasis toward the edges of the aperture. As noted above, this sidelobe level control can be applied and any reasonable sidelobe level achieved independent of aperture width, as long as this width is greater than a few wavelengths. This latter condition almost always applies in radar applications. Typically, Taylor weights are applied to the array antenna aperture to produce a sum beam while Bayliss weights are simultaneously applied to produce difference beams. Bayliss weights are similar to Taylor weights because they allow a sidelobe level to be specified, but they produce a difference beam that has the requisite gain null in the center. The resulting difference beam also has the requisite 180-degree phase change as the target angle traverses the null. When an array antenna is operated in a monopulse mode, the operation may be accomplished, in part, by division of the array into quarters. In the cases of the both the sum and difference beams, the beams are formed by coherently combining the four array quarters, but in different ways, essentially with different weight tapers. As a result, the sum beam and two difference beams are based on exactly the same size antenna apertures. When the sum of the upper two aperture quarters is subtracted from the sum of the lower two aperture quarters, an elevation angle difference beam is produced, but one that has extremely high sidelobes in a vertical plane through the center of the beam. These sidelobes are so high that it may not be clear where the difference beam mainlobes end and the sidelobe region begins. The sidelobes of this difference beam are high not because of the effective aperture size relative to the sum beam, but because the subtraction process produces an effective weighting across the vertical aperture that differs greatly from optimal Bayliss weighting. The subtraction process results in a large weight discontinuity between the two elements that straddle the center of the aperture and it is this weight discontinuity that produces the high sidelobes. Bayliss weights eliminate this discontinuity by tapering the signal emphasis not only toward the edges of the aperture as in the case of Taylor weights, but also toward the center of the aperture. Unfortunately, the simultaneous implementation of Taylor weights for the sum beam and Bayliss weights for the difference beams is expensive and results in a very complicated antenna feed structure. Low-cost radars avoid the use of Bayliss weights by simply subtracting one half of a Taylor-weighted aperture from the other. But they must then accept the high difference beam sidelobes that result. Sometimes these high difference beam sidelobes are not a problem, as in the case of an airborne radar, but when the radar is operated close to a reflecting surface they may result in severe multipath interference. Due to these high sidelobes, the elevation difference beam has relatively high gain in the direction of the undesired multipath signal. The net direct-path plus multipath target signal received by the difference beam may then differ significantly from the signal that would be received through an ideal low-sidelobe difference beam. As a result, the target elevation angle estimate derived from the perturbed difference beam signal may be severely errored.
Improved radar multipath rejection is desired.